1. Field of the Invention
This invention relates to the recovery of oil from a subterranean formation. More specifically, this invention relates to the recovery of oil from subterranean formations by injecting thereinto mutually immiscible phases where one phase is a microemulsion and another phase is an aqueous solution.
2. Description of the Prior Art
The petroleum industry has recognized for many years that only a small fraction of the original oil in place in a reservoir is expelled by natural mechanism. It is also well-known that conventional methods of supplementing natural recovery are relatively inefficient. Typically, a reservoir may retain half its original oil even after the application of currently available methods of secondary recovery. Accordingly, there is a continuing need for improved recovery methods which will substantially increase the ultimate yield of petroleum from subterranean reservoirs.
Waterflooding is by far the most widely practiced method for recovering oil from a formation after naturally occurring forces in the formation have declined in their ability to expel oil. In waterflooding, water is injected through an input well to drive oil to offset producing wells. Much of the current work in oil recovery technology has been directed toward improving the efficiency of waterflooding processes or developing alternative process.
Surface-active agents or surfactants are one class of materials which have been proposed for improving the efficiency of wateflooding processes. Much of the oil that is retained in the reservoir after a typical waterflood is in the form of discontinuous globules or discrete droplets which are trapped within the pore spaces of the reservoir. Because the normal interfacial tension between the reservoir oil and water is so high, these discrete droplets are unable to sufficiently deform to pass through narrow constrictions in the pore channels. When surface-active agents are added to the flood water, they lower the interfacial tension between the water and the reservoir oil and permit the oil droplets to deform and flow with the flood water. It is generally conceded that the interfacial tension between the flood water and the reservoir oil must be reduced to less than 0.1 dyne/cm for additional recovery.
Although conventional surfactant waterflooding may be effective in obtaining addition oil from subterranean oil reservoirs, it has a number of shortcomings which detract seriously from its value. One difficulty which has been observed in the use of surfactants in general is the tendency of the surfactants to be depleted from the injected solution. It has been demonstrated that at least a portion of the surface-active agents are adsorbed on the rock surface of the reservoir or/and a portion is physically entrapped within the pore spaces of the rock matrix. It is also known that many surfactants react with ionic substances in the water within the formation and are thereafter unable to interact at the oil/water interface to reduce the interfacial tension. This surfactant depletion can reduce oil recovery efficiency.
One method for reducing surfactant depletion and increasing oil recovery is the use of microemulsions. Microemulsions are stable, transparent or translucent mixtures of a liquid hydrocarbon, water and a surfactant. Optionally, a co-solvent such as alcohol and electrolytes may be present in the mixture. Generally, microemulsions may be oil-external, water-external or microemulsions wherein no external phase can be identified. Conventionally, a microemulsion is miscible with the crude oil in the formation and the polymer water which displaces it at least when initially injected.
In practice, a microemulsion slug is typically injected into the formation and driven towards a producing well by an aqueous slug thickened with a polymer. A problem arises in that once the microemulsion and drive slugs begin moving through the formation, for a variety of reasons, the polymer often moves more rapidly than the microemulsion components. The result is a mixing of polymer, surfactant, and other microemulsion components often causing undesired phases to appear, altering mobilities, abrubtly changing saturations, and increasing surfactant retention. Some of these problems and possible reasons thereof are discussed in detail by E. I. Sandvik and J. M. Maerker in "Application of Xanthan Gum for Enhanced Oil Recovery", ACS Symposium Series, No. 45, Extracellular Microbial Polysaccharides, edited by P. A. Sanford and A. Laskin, p. 242 (1977). Another problem that occurs is the rate that the various slugs can be injected is often a limiting factor in the rate that oil can be displaced and produced. Thus, even if the reservoir process is relatively efficient, production rates often would not be great enough to warrant a microemulsion flood, simply because the various slugs cannot be injected at a high enough rate.
One approach to the various problems found in microemulsion flooding is described in U.S. Pat. No. 3,768,560 (issued on Oct. 30, 1973 to H. J. Hill and D. R. Thigpen). There, the concentration of the polymer in the thickened drive water slug is adjusted to give a mobility as low as that of the reservoir fluids, and the concentration of the surfactant in the microemulsion slug adjusted to provide a rate of frontal advance of surfactant that equals the rate of frontal advance of polymer. While tailoring of such initial concentrations in a conventional microemulsion flooding process does, to some extent, improve the efficiency of the process, there still remains unanswered problems inherent in flooding over vast reservoir distances. That is, as the slugs move through the reservoir, due to permeability variations throughout the reservoir, various retention mechanisms (e.g. adsorption and inaccessible pore volume to polymer) act to vary chemical concentrations. Wide variations in phase behavior can still occur, with adverse mobilities and unacceptably high interfacial tensions resulting.
In a departure from conventional microemulsion flooding, wherein the microemulsion is miscible with the formation fluids at least prior to injection, it has been suggested in U.S. Pat. No. 3,885,628 (issued to Reed et al on May 27, 1975) to inject microemulsion systems which are initially immiscible with formation crude oil and formation water. This patent also suggests injecting two or more mutually immiscible phases into the oil-bearing formations to recover oil. The different phases preferably have physical and chemical properties broadly approximating those of the formation fluids. Although this approach to microemulsion flooding appears to have promise in recovering oil, surfactant and polymer adsorption and retention continue to reduce displacement efficiency. A need exists for an improved microemulsion flooding system and method for recovering oil.